1. Field of Invention
The present invention relates in general to solar cells, and more particularly, to solar cells having smooth front surfaces and both n-type and p-type electrical contacts on the backside of the cells.
2. Description of Related Art
Conventional solar cells consist of a semiconductor body having a p-type semiconductor layer, an n-type conductivity layer, an n-p or p-n semiconductor junction between these layers, a front light-receiving major surface and a back major surface. The layer adjacent to the front surface is called the emitter, and the layer adjacent to the back surface is called the base. When light energy impinges on the front light receiving surface of the cell, electrons and corresponding holes are created in both the emitter and base. For the most part, because of the presence of the semiconductor junction, electrons will be directed toward one major surface of the cell and holes toward the other major surface, resulting in a photo current density. In a typical n-p silicon semiconductor junction solar cell, electrons move to the front light receiving surface of the cell and holes toward the back surface. Typically electrical contacts are attached to the front and back surfaces of the silicon semiconductor body to collect the charge carriers. The holes are collected by the back electrical contact and electrons by the front electrical contact. In certain applications, however it is desirable that solar cells have a perfectly smooth front light receiving surface from which minimal light scatter occurs called a specular surface.
A silicon concentrator cell which has a front surface without contacts has been disclosed in an article by Sinton, Kwark, Gruenabaum & Swanson, entitled "Silicon Point Contact Concentrator Solar Cells," Proc. 18th IEEE Photovoltaic Specialists' Conference, 61-65 (1985). The front surface of the semiconductor body of the solar cell is passivated with a layer of oxide, which mitigates front surface recombination. At the backside of the semiconductor body, are disposed a plurality of p+- and n+-type conductivity regions. Both n+ and p+ electrical contacts are disposed on the back surface of the semiconductor body making contact to these regions, respectively. In operation, sunlight hits the front major light receiving surface of the solar cell generating electron-hole pairs, near the front surface. These carriers must drift as much as about 80 to 240 microns to the back surface of the cell to be collected by the electrical contacts and thereby provide useful power current. During the lifetime of the solar cell, and especially in space applications, radiation rapidly degrades this solar cell and consequently the carrier lifetime of the electrons and holes, such that fewer and fewer electrons are able to reach the back n+ contact before recombining. Therefore, efficiency of the solar cell greatly decreases over time.